Raising chemical pulp yields is an important issue for effective utilization of wood resources, i.e., for achieving effective utilization of wood resources. A polysulfide cooking process is one technology for raising the yield of kraft pulps, which are the predominant chemical pulps. A cooking liquor in a polysulfide cooking process is produced by oxidation of an aqueous alkaline solution that contains sodium sulfide, i.e., white liquor, with molecular oxygen, e.g., air, in the presence of a catalyst, e.g., active carbon, as shown by the reaction formula (1) below (Patent Document 1, Patent Document 2).
Using this method, a polysulfide cooking liquor having a polysulfide sulfur concentration of about 5 g/L can be obtained with a selectivity of about 60% and a conversion rate of about 60% on a sulfide ion basis. However, when the conversion rate is raised with this method, the thiosulfate ion, which makes absolutely no contribution to cooking, is secondarily produced in large amounts by secondary reactions as shown by the reaction formulas (2) and (3) below, and as a consequence it has been quite difficult to produce a cooking liquor containing high concentrations of polysulfide sulfur with high selectivities.[C1]4Na2S+O2+2H2O→2Na2S2+4NaOH  (1)2Na2S+2O2+H2O→Na2S2O3+2NaOH  (2)2Na2S2+3O2→2Na2S2O3  (3)
Here, polysulfide sulfur, which is also indicated by PS—S, refers to 0-valent sulfur in, for example, sodium polysulfide Na2Sx, i.e., (x−1) sulfur atoms. Further, sulfur in the polysulfide ion corresponding to sulfur with oxidation number of −2 (one sulfur atom per Sx2− or Na2Sx) and sulfide ion (S2−) are collectively referred to in this Description as Na2S— state sulfur. The liter volume unit is represented by L in this Description.
Meanwhile, Patent Document 3 discloses a method for electrolytic production of a polysulfide cooking liquor. This method is a polysulfide production method that is characterized by production of polysulfide ion through electrolytic oxidation by introducing a sulfide ion-containing solution into the anode compartment of an electrolytic bath that comprises: an anode compartment in which a porous anode is disposed, wherein the porous anode has a physically continuous three-dimensional mesh structure, at least the surface of which is composed of nickel or a nickel alloy containing nickel by at least 50 weight % (weight %=mass % here and below) and the surface area of the anode per unit volume of the anode compartment is 500 to 20,000 m2/m3; a cathode compartment in which a cathode is disposed; and a membrane that partitions the anode compartment from the cathode compartment.
With reference to a method for cleaning a membrane on which impurities have become deposited, Patent Document 4 describes a method in which the membrane of an electrolytic water conditioner provided with a membrane is made of a material that can be energized to function as a positive electrode (+electrode); at least one of the electrolytic water-conditioning electrodes is used as a negative electrode (−electrode); and an electrolytic cleaning voltage is applied to this negative electrode and the membrane positive electrode, thereby cleaning the membrane through elution of the impurities, e.g., calcium, deposited on the membrane into water.
A method is disclosed in Patent Document 5 for recovering the performance of an electrolytic bath that has an anode compartment in which a porous anode is disposed, a cathode compartment and a membrane that partitions the anode compartment from the cathode compartment, wherein this electrolytic bath produces a polysulfide sulfur-containing polysulfide through electrolytic oxidation in which a sulfide ion-containing solution is introduced into the anode compartment and an aqueous solution containing caustic soda is introduced into the cathode compartment. This performance recovery method is characterized by cleaning the anode compartment using an aqueous solution that contains at least one of an inorganic acid, a chelating agent and a scale cleaning agent.